Paste extruded insulator with air channels

ABSTRACT

The invention provides an insulated wire having a conductor and an insulator of paste extruded PTFE fine powder around the conductor. The paste extruded insulator has at least one closed continuous longitudinal air channel spaced apart from the conductor. The invention further provides a process for forming an insulator around a conductor by paste extruding lubricated PTFE fine powder in an extrusion device comprising a die, mandrel, and at least one channel-forming member. The die and mandrel form a converging chamber leading to an extrusion orifice and the channel-forming member is positioned in the orifice. The mandrel has a central bore for supplying the conductor. Lubricated PTFE fine powder is forced through the chamber and out of the exit of the orifice as a lubricated green extrudate around the conductor forming an insulator with at least one closed longitudinal air channel spaced apart from the conductor.

FIELD OF THE INVENTION

This invention relates to an insulated wire and a method formanufacturing it by paste extruding an insulator ofpolytetrafluoroethylene.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) fine powder has been used for many yearsas a wire insulator. However, despite the desirable properties of highmolecular weight PTFE, namely low dielectric constant and lowdissipation factor, other polymers have been used in forming insulationfor wires, especially in large diameter, high frequency coaxialtransmission cable because of the processing limitations of PTFE.

Polytetrafluoroethylene (PTFE) fine powder is a type of PTFE that ismade by aqueous dispersion polymerization, followed by coagulation ofthe dispersion and drying of the resultant coagulated solids to obtainthe fine powder. Because the PTFE fine powder does not flow in the meltcondition sufficiently to enable melt processing, the powder has beenfabricated into articles and coated onto wire by an extrusion methodwhich does not require melt flow. This extrusion method is known aspaste extrusion and is described for example in U.S. Pat. No. 2,685,707.In paste extrusion, a paste extrusion composition is formed by mixingPTFE fine powder with an organic lubricant which has a viscosity of atleast 0.45 centipoise at 25° C. and is liquid under the conditions ofsubsequent extrusion. The PTFE soaks up the lubricant, resulting in adry, pressure coalescing paste extrusion composition that is alsoreferred to as lubricated PTFE fine powder. During paste extrusion whichis typically performed at a temperature of 20 to 60° C., the lubricatedfine powder is forced through a die to form a lubricated greenextrudate. The lubricated green extrudate is then heated, usually at atemperature of 100 to 250° C., to make volatile and drive off thelubricant from the extrudate. In most cases, the dried extrudate isheated to a temperature close to or above the melting point of the PTFE,typically between 327° C. and 500° C., to sinter the PTFE.

Cables capable of transmitting high frequency electromagnetic radiationare especially useful for communications in the radio bandwidth in areaswhere structures may inhibit signal transmission, such as in or aroundbuildings, tunnels, or garages. Likewise, such cables also have use inunobstructed areas, but where there is a need for precisely controlledsignal levels that must be distributed over distances without theinterference of other nearby signals.

High frequency transmission cable requires insulation with the lowestpossible dielectric constant ε_(T) and the lowest possible dissipationfactor tan δ to obtain the desired attenuation characteristics. Asdisclosed in U.S. Pat. No. 5,922,155, a dielectric constant of less thanabout 1.8 is desirable. Insulation material such as fluoropolymers andpolyethylene have dielectric constants close to about 2. As taught inU.S. Pat. No. 3,771,934 and 5,922,155, air cells can be introduced intoinsulation made of melt-flowable polymers to reduce the dielectricconstant.

PTFE has the lowest loss (dissipation factor) of most commonly usedpolymers. So while other polymers such as polyethylene or melt-flowablefluoropolymers, e.g., PFA [TFE/perfluoro(alkyl vinyl ether) copolymer]and FEP (TFE/hexafluoropropylene copolymer), are easier to form by usingmelt extrusion techniques, their loss is not as low as PTFE. Forexample, at a frequency of 1 MHz, the dissipation factor of PTFE is morethan two times lower than PFA and more than three times lower thanpolyethylene or FEP. Nonetheless, PTFE has been difficult to form intolarge diameter insulators for wire because of difficulties in drying thegreen extrudate to remove the lubricant from thick structures of pasteextruded PTFE fine powder. Large diameter PTFE insulators have also beenexpensive because of the quantity of PTFE fine powder required.

Another significant requirement for high frequency communication cableis noncombustibility that fluoropolymers can provide but structures ofpolyethylene, polypropylene or polyvinyl chloride cannot. PTFE has thehighest stability to heat among the fluoropolymers.

An insulated wire of paste extruded PTFE with lower dielectric constant,with lower dissipation factor and which requires a smaller quantity ofPTFE fine powder for its manufacture would be highly desirable for usein high frequency cables.

BRIEF SUMMARY OF THE INVENTION

The invention provides an insulated wire having a conductor and aninsulator of paste extruded PTFE fine powder around the conductor. Thepaste extruded insulator has at least one closed continuous longitudinalair channel spaced apart from the conductor. Preferably the insulator isdried and then sintered. In a preferred embodiment, the insulated wireis part of a coaxial cable.

The invention further provides a process for forming an insulator arounda conductor by paste extruding lubricated PTFE fine powder in anextrusion device comprising a die, mandrel, and at least onechannel-forming member. The die and mandrel form a converging chamberleading to an extrusion orifice and the channel-forming member ispositioned in the orifice. The mandrel has a central bore for supplyingthe conductor. Lubricated PTFE fine powder is forced through the chamberand out of the exit of the orifice as a lubricated green extrudatearound the conductor forming an insulator with at least one closedlongitudinal air channel spaced apart from the conductor.

In one preferred embodiment, the channel forming-member is supportedupstream of the orifice. In another preferred embodiment, the extrusiondevice has multiple channel-forming members which form multiple closedlongitudinal channels spaced apart from each other. In a more preferredembodiment, the lubricated PTFE fine powder is forced around thechannel-forming member into a space between the conductor and thechannel forming member pressure coalescing the lubricated PTFE finepowder into the lubricated green extrudate around the conductor.Preferably, the extrusion device has a high shear region formed at thejuncture of the chamber and the orifice which causes substantialfibrillation and the pressure coalescing is performed upstream of thehigh shear region. Preferably, the lubricated green extrudate is driedand, most preferably, gas is circulated though the air channel of thelubricated green extrudate to facilitate lubricant removal duringdrying. For most applications, it is also preferred to sinter the driedgreen extrudate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial longitudinal cross-section of the extrusion deviceused in this invention.

FIG. 2 is an enlarged perspective view of one embodiment of a mandreltip employed in the extrusion device of FIG. 1.

FIG. 3 is a partial perspective view of an insulator formed using themandrel tip illustrated in FIG. 2.

FIG. 4 is an enlarged perspective view of a second embodiment of amandrel tip for use in the extrusion device of FIG. 1.

FIG. 5 is a partial perspective view of an insulator formed using themandrel tip illustrated in FIG. 4.

FIG. 6 is a partial perspective cut away view of a coaxial cable withinsulated wire having an insulator and longitudinal air channels madeaccording to this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a process for paste extruding lubricated PTFEfine powder around a conductor forming an insulator having at least oneclosed continuous longitudinal air channel spaced apart from theconductor. Air channels in the insulator provide a means for aiding theremoval of volatilized lubricant from the green extrudate duringmanufacture and for reducing the dielectric constant of the sinteredproduct which is especially useful for high frequency communicationcables.

The process of paste extrusion of lubricated PTFE fine powder may beunderstood with reference to FIG. 1 wherein there is shown an extrusiondevice 1 having a barrel 2 and a die 4. Inside device 1 there is defineda chamber 3 for supplying lubricated PTFE which leads to extrusionorifice 5. Positioned within the chamber is mandrel 10 for shaping thePTFE and positioned within orifice 5 is at least one channel-formingmember 8. The die and the mandrel cause chamber 3 to converge in thearea designated as convergence zone 15. Channel-forming member 8 issupported by extrusion device 1, preferably upstream of the orifice 5,and is most preferably supported by the mandrel 10. Preferably,channel-forming member 8 is recessed in orifice 5 upstream of exit 7 ofthe orifice. Mandrel 10 has a central bore 11 for supplying conductor12. Conductor 12 can be either a single strand or a multi-strand wire.Further as illustrated in FIG. 1, the convergence zone has a high shearregion 19 at the juncture of chamber 3 and orifice 5.

Lubricated PTFE fine powder, usually as a charge shaped under pressureto conform to chamber 3, is placed in chamber 3 and is then pasteextruded by a ram 9 positioned in chamber 3 at the end oppositeextrusion orifice 5 which moves toward extrusion orifice 5 to force thelubricated PTFE fine powder through extrusion orifice 5 and out orificeexit 7 around conductor 12. Preferably, the mandrel 10 andchannel-forming member 8 are positioned so that the lubricated PTFE finepowder is forced around channel-forming 8 member and into a spacebetween conductor 12 and channel-forming member 8 and is subjected topressure coalescing in convergence zone 15, preferably upstream of highshear region 19, forming a lubricated green extrudate around theconductor.

Lubricated PTFE fine powder comprises PTFE primary particles which,during paste extrusion, are deformed into small interconnectedfiber-like structures, called fibrils and thus the fine powder is saidto fibrillate. The lubricated PTFE fine powder is fibrillated by shearforces as it is forced through the extrusion device 1. Substantialfibrillation occurs in high shear region 19 at the juncture of thechamber 3 and the orifice area 5 between channel forming member 8 andthe walls of orifice 5. These fibrils are primarily oriented in thedirection of the paste extrusion which means that after extrusion, thelubricated extrudate is especially strong in the extrusion direction. Inthe preferred process of this invention the lubricated PTFE fine powderis pressure coalesced around the conductor 12 prior to substantialfibrillation in the high shear zone 19. Premature fibrillation canhinder adherence of the extrudate to the conductor and can create areasof weakness in the extrudate due to poor knitting of polymer in thoseareas.

It is further preferred that the speed at which the conductor issupplied approximates the speed of the lubricated PTFE as it is beingforced through the orifice, again insuring better adhesion of theinsulator to the conductor.

One embodiment of channel-forming member 8 used in this invention isillustrated in FIG. 1 and may be seen in perspective in FIG. 2. In FIG.2, the channel-forming member 8 is provided by a mandrel tip 6 whichincludes a connector element 18, such as a threaded connection forattachment to the mandrel 10, which supports the channel-forming member8 positioned in orifice 5. As illustrated, the mandrel tip 6 of thisembodiment can be described as a cylinder with three radial spaces 23dividing the cylinder into three pie-shaped segments 20 serving aschannel-forming members. The radial spaces 23 are larger upstream buttaper to a smaller size near the exit 7 or the orifice 5. The mandreltip 6 also includes and a central space 21 for accommodating conductor12 (not shown) being fed from mandrel bore 11. Each pie-shaped segmenthas a bore 22 running the length of the mandrel tip 6 to provide fluidcommunication with mandrel bore 11 through which a vacuum is drawn or,alternatively, air is pumped during extrusion to aid in removinglubricant during drying as will be described in more detail hereinafter.

The insulated wire 30 shown in FIG. 3 having a conductor 12 and aninsulator 16 of paste extruded PTFE, is formed using extrusion device 1with the mandrel tip 6 shown in FIG. 2. The insulator has multipleclosed continuous longitudinal air channels 17 spaced from each otherand are disposed symmetrically about the conductor. The insulator 16 isformed as the lubricated PTFE fine powder is forced around the pieshaped segments 20 of mandrel tip 6 and into the space between theconductor and the mandrel tip forming three pie-shaped closed continuouslongitudinal air channels 17 spaced apart from the conductor 12.Pressure coalescence of the extrudate occurs preferably as the PTFEpasses through the radial spaces 23 to the central space 21 and thenenters the space between the conductor and the channel-forming memberupstream of the high shear region 19 shown in FIG. 1.

An alternative embodiment of the mandrel tip 40 for use in thisinvention is illustrated in perspective in FIG. 4. This mandrel tip 40also has a connector element 44 connecting to a mandrel tip body 45 witha tapering end 46. The channel forming members of mandrel tip 40 iscomprised of four tubes 42 symmetrically spaced around central tube 43for accommodating a conductor (not shown) being fed from the mandrelbore 11. The four tubes 42 and central tube 43 extend from the taperingend 46 of the mandrel tip body 45 with the central tube extending ashorter distance from the tapering end 46 of the mandrel tip body 45than the tubes 42. The mandrel tip body 46 has appropriate passages (noshown) so that the four tubes 42 are in fluid communication with themandrel bore 11 so that, during extrusion, a vacuum can be drawn throughthe tubes, or alternatively air is forced in the tubes, to aid inremoving lubricant during drying as will be described in more detailhereinafter.

The insulated wire 50 shown in FIG. 5 having a conductor 12 and aninsulator 51 of paste extruded PTFE, is formed using an extrusion devicewith the mandrel tip shown in FIG. 4. The insulator has multiple closedcontinuous longitudinal air channels spaced from each other and aredisposed symmetrically about the conductor. The insulator 51 is formedas the lubricated PTFE fine powder is forced around the tubes of thechannel-forming member and into the space between the conductor and thetubes forming an insulator 51 with four circular closed continuouslongitudinal air channels 52 spaced apart from the conductor 12.Pressure coalescence of the extrudate occurs preferably as the PTFEenters the space between the conductor and the tubular channel-formingmember upstream of the high shear region of the extrusion device.

Paste extrusion in accordance with the process of the invention ispreferably carried out at a temperature of 20 to 60° C., thoughextrusion temperatures outside this range can also be used. Thelubricated green extrudate is then heated, usually at a temperature of100 to 250° C., to volatilize and drive the lubricant from theextrudate, drying the extrudate.

In accordance with a preferred form of the invention, a gas suitable forcarrying the volatilized lubricant, e.g., air, is circulated through theair channels of the extrudate during drying. This is advantageouslyaccomplished by introducing gas, or alternatively, applying a vacuum tothe mandrel bore 11 which causes gas flow though the bores 22 in mandreltip 6 depicted in FIG. 2 or the tubes 42 in mandrel tip 40 depicted inFIG. 4. This gas flow causes gas flow though the newly-formed airchannels in the green extrudate and the flow is continued though theportion of the extrudate being dried. The gas flows continue to the endof the extrudate where the air channels operate as vents, either to ventgas (and volatilized lubricant) being pumped or to admit gas being drawninto the insulator. Drying times are shortened substantially by the gasflow through the extrudate being dried. Previously, the drying steplimited the size of articles that could be made from high molecularweight PTFE in commercially viable processes. Prior to this invention,only coated wire with small diameter insulators could be made using PTFEbecause large diameter insulators would have thick walls hindering thevolatilization of the lubricant. With this invention however,longitudinal air channels and a reduced mass of PTFE facilitatelubricant extraction permitting the formation of large diameter cablesfrom high molecular weight PTFE.

Insulated wire having a wide range of diameters, e. g., about 2 mm toabout 125 mm, can be made using the process in accordance with theinvention. The invention is advantageously employed to make insulatedwire with diameters greater than about 8 mm, more preferably greaterthan 10 mm, even more preferably greater then about 15 mm, and mostpreferably, greater than about 20 mm. An especially preferred range isabout 10 mm to about 50 mm. Such large diameters can be madeeconomically though use of the invention due to the decreased dryingtimes.

After drying, most applications the extrudate is preferably sintered inan oven by heating to temperature between 327° C. and 500° C. for aperiod sufficient to strengthen the insulator around the wire.

The invention further provides a coaxial cable with the insulated wiremade by the process the described above. A coaxial cable generallyincludes an inner conductor, an outer conductor concentrically arrangedaround the inner conductor, an insulator made of paste extruded PTFEbetween the conductors and an outer protective sheath. FIG. 6illustrates a coaxial cable 60 having an insulated wire made using theextrusion device of FIG. 1 and the mandrel tip 40 described in FIG. 4.Shown is a central core of a plurality of stranded conductor wires 61,an insulator 62 of paste extruded PTFE fine powder around the conductorwires having four closed continuous longitudinal tubular air channels 63spaced apart from the conductor. Around insulator 62 is a braided orwoven outer conductor 64 and a protective outer sheath 65.

In order to obtain the attenuation at high frequencies required bycoaxial cables, the dielectric insulator is should have a dielectricconstant as close to 1 as possible. Fluoropolymers are known for theirlow dielectric constants. The dielectric constant of unsintered PTFE isabout 1.7 and that of sintered PTFE is about 2. The incorporation of airchannels in the insulator reduces the dielectric constant to usefulranges for coaxial cables transmitting high frequency electromagneticradiation.

Further, PTFE has the lowest loss (dissipation factor) of most commonlyused polymers. So while other polymers such as polyethylene ormelt-flowable fluoropolymers, e.g., PFA [TFE/perfluoro(alkyl vinylether) copolymer] and FEP (TFE/hexafluoropropylene copolymer), areeasier to form by using melt extrusion techniques, their loss is not aslow as PTFE. For example, at a frequency of 1 MHz, the dissipationfactor of PTFE is more than two times lower than PFA and more than threetimes lower than polyethylene or FEP. The process of the presentinvention makes it possible to take advantage of the desirableproperties of PTFE fine powder, i.e., low dielectric constant and lowloss and form PTFE into insulation for large diameter high frequencycable. Further, the high heat stability of PTFE makes this fluoropolymermore desirable than other commonly used polymers.

What is claimed is :
 1. An insulated wire comprising a conductor and aninsulator of paste extruded PTFE fine powder pressure coalesced aroundsaid conductor, wherein said insulator has at least one closedcontinuous longitudinal air channel spaced apart from said conductor. 2.The insulated wire of claim 1 wherein said insulator has multiple closedcontinuous longitudinal air channels spaced from each other.
 3. Theinsulated wire of claim 2 wherein said longitudinal air channels aredisposed symmetrically about the conductor.
 4. The insulated wire ofclaim 1 wherein said insulator has been sintered.
 5. A coaxial cablecomprising the insulated wire of claim
 1. 6. The insulated wire of claim1 wherein said insulator has a diameter of greater than about 8 mm. 7.The insulated wire of claim 1 wherein said insulator has a diametergreater than about 10 mm.
 8. A lubricated green extrudate of pasteextruded PTFE fine powder pressure coalesced around a conductor, saidlubricated green extrudate comprising organic lubricant, wherein saidlubricated green extrudate has at least one closed continuouslongitudinal air channel spaced apart from said conductor.